This invention relates to optical waveguides and, more particularly, to hollow waveguides based on optical confinement by Bragg reflection that are fabricated with integrated circuit (IC) technology.
Waveguiding of light is typically based on refractive index contrast for optical confinement. For example, the vast majority of optical fibers are dielectric fibers comprising a core of high refractive index material surrounded by a cladding of lower index material whereby light is guided within the fiber by total internal reflection of the guided light at the core-clad interface. In particular, silica-core fibers are ideal for propagation of telecommunications signals at the near-infrared absorption minimum of silica.
Conventional dielectric fibers can have disadvantages for specialized applications. Fiber attenuation can result from absorption and scattering of the guided light by the core material. Silica and most other materials become highly absorbing at longer wavelengths, limiting the far-infrared transmission through most solid-core fibers. Furthermore, Rayleigh scattering in solid core materials increases rapidly at shorter wavelengths. Dielectric fibers typically have small core-clad refractive index contrast and consequent large critical angle for total internal reflection. As a result, dielectric fibers can suffer large bending losses when the angle at which the light hits the core-clad interface falls within the critical angle at small bending radii. Thus, conventional dielectric fibers cannot bend light around sharp turns, important for optical integrated circuits and other microphotonic applications. The refractive index contrast of the waveguide can be improved with higher index semiconductor core materials, enabling tighter bending radii. However, it becomes more difficult to effectively couple light into the waveguide when the refractive index of the core material is increased. This coupling problem can make the assembly and packaging of integrated microphotonic systems difficult.
Recently, interest has grown in hollow- and liquid-core waveguides, primarily for medical and industrial applications. With hollow-core waveguides, the solid core is replaced by a non-absorbing gas or vacuum. The cladding material generally has a refractive index greater than the core material such that the wave is guided by reflections at the core-clad interface. Attenuation due to core material absorption can be low and such hollow waveguides can have large damage thresholds and high power capacities due to the absence of a core material. Furthermore, hollow waveguides can have low insertion loss, since end reflections from a solid core are eliminated.
However, metal-clad hollow waveguides, in particular, can be leaky due to imperfect wall reflectivity resulting from absorption and diffuse scattering by the metal clad at infrared and visible wavelengths. Scattering due to surface roughness is further accentuated because reflections are at near-grazing incidence. Multiple imperfect reflections can result in large transmission losses, thereby favoring large cross-section hollow waveguides. Likewise, hollow waveguides can suffer large bending losses due to mode coupling and the increased number of reflections off of the outer and inner walls of the waveguide with tight bends. It has been observed that the reflectivity of metal-clad hollow waveguides can be improved by coating the internal metallic guide surface with a thin, less-conductive dielectric cladding layer. However, only relatively large dielectric-coated metal hollow waveguides have been fabricated. Furthermore, such dielectric-coated metal waveguides can still suffer relatively large bending losses and transmission losses due to interaction of the guided wave with the underlying metallic layer. Harrington et al. xe2x80x9cReview of hollow waveguide technology,xe2x80x9d SPIE 2396, 4 (1995).
Bragg fibers, built on the principle of the cylindrical multilayer dielectric mirror, have been proposed for low loss broadband guiding of light in air. Analysis has shown that confined modes can exist in a Bragg fiber comprising a low-index core (for example, air) surrounded by a cladding of alternating high- and low-refractive-index layers. Yeh et al. xe2x80x9cTheory of Bragg Fiber,xe2x80x9d J. Opt. Soc. Am. 68, 1196 (1978). These Bragg fibers are predicted to have low transmission loss and can have a large single mode volume. Recently, the theory of the Bragg fiber has been extended to include omnidirectional waveguides that exhibit strong reflectivity over a range of incident angles with appropriate choice of dielectric layers, allowing for guiding light around sharp bends. Fink et al. xe2x80x9cA Dielectric Omnidirectional Reflector,xe2x80x9d Science 282, 1679 (1998).
Waveguiding in a Bragg fiber comprising alternating thin layers of polymer and tellurium on the inside of a rubber tube has recently been demonstrated. Fink et al. xe2x80x9cGuiding Optical Light in Air Using an All-Dielectric Structure,xe2x80x9d J. Lightwave Tech. 17, 2039 (1999). This large diameter Bragg fiber exhibited strong omnidirectional reflectivity and good transmission around a relatively small radius bend for guided light in the wavelength range of 10 to 15 micrometers. However, the Bragg fiber described by Fink et al. is not fabricated using IC technologies and, therefore, does not use semiconductor-compatible materials and is limited to guiding longer wavelength light.
Bragg waveguides, with multilayer dielectric cladding, may be attractive for microphotonic applications. However, a need remains for small diameter Bragg waveguide that can transmit light at wavelengths of use with optical integrated circuits and that can be fabricated with semiconductor-compatible technologies and materials.
The present invention comprises a microfabricated Bragg waveguide and a method for fabricating the Bragg waveguide. The microfabricated Bragg waveguide has a number of attractive features for use in microphotonics applications. It is designed to allow modest radiation losses for both TE and TM polarizations, thus leading to a waveguide of general utility. The increase in the mode size and low insertion loss afforded by the propagation of light in air may greatly improve coupling efficiency to optical components, a critical issue for integrated microphotonics. Coupling may also be improved resulting from the absence end reflections, which can be a difficulty with silica fibers requiring highly polished end facets. Light propagation in an air-core waveguide may also reduce some material dispersion effects that are inherent with solid-core fibers. The microfabricated Bragg waveguide of the present invention can be fabricated with IC technologies using semiconductor-compatible materials. This enables material and fabrication flexibilities not possible with prior art Bragg fibers. For example, the microfabricated Bragg waveguides can have arbitrary cross-section and very small core size. The small core size enables guided waves with only a few modes. The reduced size further enables integration with microelectromechanical systems (MEMS) actuation schemes and smaller and less complex optical integrated circuits.
The present invention provides a microfabricated Bragg waveguide of semiconductor-compatible materials. The microfabricated Bragg waveguide can be a channel or fiber having a hollow core for the propagation of an optical guided wave therein. The Bragg waveguide further comprises a multilayer dielectric cladding disposed on at least one wall of the fiber or the inner wall of the channel, the cladding comprising at least one alternating layers of a first semiconductor-compatible dielectric material having a high index of refraction and a second semiconductor-compatible dielectric material having a lower index of refraction, such that the thicknesses of the alternating layers are carefully chosen to minimize radiation loss.
The present invention further comprises a method for fabricating a Bragg channel waveguide, comprising coating a top surface of a substrate with a mask layer of a structural material, forming an opening in the structural mask layer, etching a trench in the substrate through the opening in the structural mask layer, and coating the inner wall of the trench with a multilayer dielectric cladding.
The present invention further provides a method for fabricating a Bragg fiber, comprising forming a trench in a substrate, coating the inner wall of the trench with a first layer of a structural material, filling the structural material-lined trench with a sacrificial material to leave an exposed deposit surface, coating the deposit surface of the sacrificial material with a second layer of the structural material, removing the sacrificial material to leave a hollow fiber in the trench, removing the substrate to leave a hollow fiber of the structural material, and coating at least one wall of the hollow fiber with a multilayer dielectric cladding.
Alternatively, the Bragg fiber can be fabricated by forming a mandrel of a sacrificial material, coating the surface of the mandrel with a multilayer dielectric cladding, and removing the sacrificial material to leave a hollow tube of the multilayer dielectric cladding.